Strokes happen when blood flow to part of the brain is cut off, either by a clot blocking an artery or by a blood vessel bursting and bleeding into brain tissue. Every minute a stroke goes untreated, roughly 1.9 million neurons are destroyed, along with 14 billion connections between them. Globally, about 11.9 million strokes occur each year, making it one of the leading causes of death and disability worldwide.
The Two Main Types of Stroke
About 85% of strokes are ischemic, meaning a blood clot blocks an artery supplying the brain. The other 15% are hemorrhagic, caused by a blood vessel that ruptures and bleeds. The underlying mechanism is completely different for each type, but both starve brain cells of oxygen and cause permanent damage within minutes.
How Blood Clots Cause Ischemic Strokes
Ischemic strokes start with a clot forming either inside a brain artery or somewhere else in the body before traveling to the brain. The process begins when the inner lining of a blood vessel becomes damaged, often from years of high blood pressure, high cholesterol, or diabetes. Platelets rush to the damaged area and begin clumping together. As they accumulate, they trigger a chain reaction that converts a protein in your blood into sticky, thread-like fibers. These fibers weave through the clump of platelets to form a dense mesh, essentially a plug that can partially or completely block blood flow.
When this clot forms directly inside a narrowed brain artery (a thrombotic stroke), it tends to build up gradually at the site of fatty plaque deposits. When the clot forms elsewhere, typically in the heart, and then breaks loose and travels to the brain (an embolic stroke), the blockage is sudden. Either way, the brain tissue downstream of the blockage loses its oxygen supply and begins dying.
How Bleeding Causes Hemorrhagic Strokes
Hemorrhagic strokes occur when a weakened blood vessel in or around the brain ruptures. The two main scenarios are an aneurysm, which is a balloon-like bulge in an artery wall that bursts, and an arteriovenous malformation (AVM), a tangle of abnormal blood vessels that can leak or rupture. When bleeding happens on the brain’s surface between the brain and its protective covering, it’s called a subarachnoid hemorrhage, most often caused by a ruptured aneurysm.
The damage from a hemorrhagic stroke is twofold. The blood that pools outside the vessel puts direct pressure on surrounding brain tissue, and the areas that were being supplied by that vessel lose their blood flow. Hemorrhagic strokes are less common than ischemic strokes but tend to be more deadly.
High Blood Pressure: The Biggest Risk Factor
Chronic high blood pressure is the single most important risk factor for both types of stroke. It damages artery walls over time, making them stiffer, narrower, and more prone to both clotting and rupturing. In people who also have atrial fibrillation, an irregular heart rhythm that already raises stroke risk, poor blood pressure control increases the likelihood of stroke by an additional 50%.
The reason high blood pressure is so dangerous is that it works silently. Most people feel perfectly fine even as their arteries are being gradually weakened. Years of elevated pressure promotes the buildup of fatty deposits in artery walls, creates the conditions for clot formation, and weakens the small vessels deep in the brain that are most vulnerable to bursting.
Other Major Risk Factors
Atrial fibrillation deserves special attention. When the upper chambers of the heart quiver instead of contracting normally, blood pools and clots can form. Those clots can then travel directly to the brain. People with atrial fibrillation have a significantly higher baseline stroke risk even without other contributing factors.
Diet plays a measurable role. A study following residents of northern Manhattan found that people consuming 4,000 milligrams or more of sodium per day had 2.6 times the stroke risk compared to those eating 1,500 milligrams or less. For every additional 500 milligrams of daily sodium, stroke risk climbed by 17%. For reference, a single fast-food meal can easily contain 2,000 milligrams.
Smoking damages blood vessel linings directly, accelerates plaque buildup, and makes blood more likely to clot. Diabetes has a similar effect on blood vessels, particularly the smaller ones. Obesity, physical inactivity, and heavy alcohol use all compound these risks. The important thing to understand is that these factors rarely act alone. They stack on top of each other, and the more you have, the higher your cumulative risk.
Strokes in Younger Adults
While stroke risk increases sharply with age, strokes in people under 60 are not rare, and they often have different causes. About 3 in 10 strokes are classified as cryptogenic, meaning no obvious cause is found. When doctors investigate unexplained strokes in younger patients, they frequently discover a patent foramen ovale (PFO), a small hole between the upper chambers of the heart left over from fetal development. Up to half of people under 60 with unexplained strokes have one.
A PFO normally causes no problems, but it can act as a shortcut for blood clots. Instead of being filtered out by the lungs, a clot can slip through the opening and travel directly to the brain. Other causes of stroke in younger people include tears in the artery walls of the neck (which can happen from sudden head movements, chiropractic manipulation, or even roller coasters) and blood-clotting disorders.
Warning Signs: TIAs Before a Full Stroke
Many strokes are preceded by a transient ischemic attack, sometimes called a “mini-stroke.” A TIA produces the same symptoms as a full stroke, including facial drooping, arm weakness, slurred speech, vision changes, and loss of balance. The difference is that TIA symptoms usually resolve within minutes, and most disappear within an hour. Rarely, they can last up to 24 hours.
A TIA is not a harmless event. About 1 in 3 people who experience a TIA will eventually have a full stroke, and roughly half of those strokes happen within the following year. TIAs most often occur in the hours or days leading up to a major stroke, making them a critical warning window.
Recognizing a Stroke as It Happens
The acronym BE FAST covers the signs to watch for:
- Balance: sudden loss of balance or coordination
- Eyes: vision loss in one or both eyes, or sudden double vision
- Face: drooping on one side of the face
- Arms: weakness in one arm or leg (if you raise both arms and one drifts downward, that’s a red flag)
- Speech: slurred words or difficulty understanding others
- Time: call emergency services immediately
Speed matters more with stroke than almost any other medical emergency. With nearly 2 million neurons lost every minute, the difference between calling for help immediately and waiting an hour can be the difference between a full recovery and permanent disability. Treatments for ischemic stroke can dissolve or remove clots, but they work best within the first few hours.